var idmap = new Array(new Array("H", "K", "N", "G", "O", "B", "E", "A", "R", "S"),
                      new Array("M", "Y", "S", "Q", "L", "R", "U", "8", "E", "Z"));

function userid2eid(form)
{
  // Quick sanity check
  var expr = /^[0-9]+$/;
  if(!form.userid.value.match(expr)) {
    form.eid.value = "Bad input";
    return;
  }

  var eid = "";
  for (i = 0; i < form.userid.value.length; ++i) {
    var c = form.userid.value.charAt(i) - '0';
    eid = eid + idmap[i & 1][c];
  }

  form.eid.value = eid;
}

function eid2userid(form)
{
  // Quick sanity check
  for (i = 0; i < form.eid.value.length; ++i) {
    if (idmap[i & 1].indexOf(form.eid.value.charAt(i)) == -1) {
      form.userid.value = "Bad input";
      return;
    }
  }

  userid = "";
  for (i = 0; i < form.eid.value.length; ++i) {
    var n = idmap[i & 1].indexOf(form.eid.value.charAt(i));
    userid += n;
  }

  form.userid.value = userid;
}